1. Field of the Invention
This invention relates to navigation using a global positioning system (GPS) receiver which receives information from an earth-orbiting satellite. The invention particularly applicable to making an initial estimate of the position of such a receiver.
2. Description of the Related Art
Although it has broader applicability, embodiments of the present invention arose in the continuing effort to improve the process by which a GPS receiver acquires signals transmitted by a constellation of satellites orbiting the earth. The satellite signals are necessary for navigation using the GPS. When a GPS receiver is turned on and begins to receive radio signals, it must begin searching these signals for transmissions from the GPS satellites. At any given location on the earth, the receiver can only receive signals from those satellites which are visible at that location. Because the satellite signals typically have a low signal-to-noise ration, acquisition of particular satellites can be improved if the visible satellites which are transmitting signals can be determined.
This determination, however, is not immediately apparent from the radio signals detected by the receiver. Typically, GPS receivers are capable of making a determination as to which unobscured satellites are transmitting signals by evaluating the satellite signal outputs relative to the location of the receiver itself. However, the receiver often does not know its location even approximately, in which case the receiver cannot use a known location to determine which satellites are visible.
Thus, the process of finding the signals from visible satellites begins with finding a signal from one of them and then using the identity of that satellite and its transmitted information (associated with a pseudo-random code) to select which of the other satellites' transmissions to search for next. This process is continued until transmissions are received from the minimum number of GPS satellites necessary for navigation. Generally, four satellites are required for three-dimensional tracking.
However, acquiring the necessary satellite signals from a cold start, that is, when the receiver location and/or time are unknown, is a relatively time consuming process. For example, it may require up to fifteen minutes for satellite acquisition in commercial GPS receiver units currently on the market. As can be understood, there is a substantial commercial advantage in finding a way to reliably perform this acquisition with significantly increased speed.
One approach used to improve satellite signal acquisition is to acquire a first signal, then make an initial estimate of the region of the earth in which the receiver is located by calculating a "pseudo range" from the satellite. The pseudo range describes the measurement of range from the receiver to the satellites using an imprecise clock. The pseudo range, however, is an inexact range value due to a bias of fixed magnitude in each range estimate attributable to the clock error.
The receiver then searches for signals from satellites known to be visible from the initially-estimated region of probable location. Upon acquisition of another signal, another calculation of pseudo range is made to define a narrower region of location of the receiver, and so on. As the region of location grows smaller, the probability of finding additional satellite signals improves. As mentioned above, however, the pseudo range approach has an inherent drawback. The calculated value of pseudo range is affected by drift in the internal clock of the GPS receiver. If the drift is large, the estimated region of location of the receiver may be unreasonably large, and would be correspondingly uncertain. Such a result would be of diminished value.